1. Field of the Invention
The present invention relates, in general, to optical modulation schemes and, more particularly, to an apparatus and method for performing optical differential phase shift keying modulation, which transmits a phase difference between adjacent bits as information.
2. Description of the Related Art
In a current optical transmission system, a Non-Return-to-Zero (NRZ) modulation scheme has been generally used. However, with the increase of the data rate of the optical transmission system, the limitations of the NRZ modulation scheme have appeared. Therefore, a new modulation scheme has been developed. A Differential Phase Shift Keying (DPSK) modulation scheme is one of new modulation schemes.
Generally, a DPSK modulation scheme, which transmits information using the phase of a transmitted carrier signal, includes information in a phase difference between adjacent bits and transmits the information. It is well known that DPSK modulation can further ensure a margin of 2 to 3 dB in terms of optical Signal to Noise (S/N) ratio compared to the NRZ modulation.
Methods of implementing DPSK modulation can be classified into three types: A first type is to use a phase modulator, in which a modulating signal is applied to an optical phase modulator to maintain the constant intensity of a DPSK modulated signal and change only the phase thereof; a second type is to generate a Return-to-Zero (RZ) pulse signal and vary the phase of the pulse signal using a phase modulator or a Mach-Zehnder intensity modulator, in which the intensity of an optical signal has an RZ-format and has opposite phases depending on a modulating signal; and a third type is to apply an electrical modulating signal to a Mach-Zehnder intensity modulator, in which the phase of a DPSK modulated signal changes and the intensity thereof rapidly changes in a transition region when the state of the applied modulating signal changes.
In the above-described third DPSK modulation scheme, a single Mach-Zehnder modulator is used. In this case, a voltage of 2Vπ centered at a transmission null point is applied to the Mach-Zehnder modulator, thus allowing the phases of the modulated signal to be opposite to each other at the signal levels “1” and “0” thereof. Such a conventional modulation scheme is implemented to perform coding using a pre-coder, amplify the coded signal using a high-speed driver, and immediately apply the amplified signal to an optical modulator, thus obtaining an optical DPSK modulated signal. All electrical/optical devices used in this case use the entire bandwidth corresponding to a data rate. However, if the entire bandwidth corresponding to the data rate is used as in the case of the conventional scheme, there are problems in that, in the case of high-speed transmission in Wavelength Division Multiplexing (WDM) optical transmission, the spectrum of a modulated optical signal is widened, so that the modulated optical signal may be sensitive to dispersion and it is not possible to narrow an interval between channels.
In the meantime, a method, which generates a signal strong at chromatic dispersion and non-linearity by allowing a DPSK signal to pass through a Mach-Zehnder interferometer and then forming a RZ signal, is disclosed in U.S. Pat. Publication No. US2003/0002121 entitled “Optical transmitter and optical transmission system”. Further, a method of transmitting an RZ-DPSK signal to a distributed managed transmission channel to reduce a non-linear phenomenon is disclosed in U.S. Pat. Publication No. US2003/007216 entitled “Long haul transmission in a dispersion managed optical communication system”. Further, a method of reducing an intra-channel non-linear phenomenon and the influence of Polarization Mode Dispersion (PMD) by applying a polarization interleaving method to a DPSK modulation scheme is disclosed in U.S. Pat. Publication No. US2003/0090768 entitled “Long haul optical communication system”. However, the conventional methods are problematic in that influences due to crosstalk between adjacent channels and dispersion, occurring when high-speed transmission is performed using the DPSK modulation, cannot be solved.